1. Field of the Invention
This invention relates to a mounting structure for external wallboards wherein an external wallboard formed from a material containing cement is mounted by screws or nails.
2. Description of the Prior Art
External wallboards formed from a material containing cement (hereinafter, "cement external wallboards") usually have decorative irregular patterns formed on surfaces thereof. The demand for these wallboards have recently been increased because of their high decorativeness. Two methods are known for mounting the external wallboards on wall backings, that is, a first method using fittings and a second method using screws or nails. In the first method, solid ends of the wallboard are fitted in the fittings secured to the wall backings by screws or nails to be thereby held in position. In this construction, displacement is allowed between the solid ends of the wallboard and the fittings. This further permits the displacement in the framed construction of a building due to an earthquake force or wind pressure load and the displacement of the external wallboard caused by expansion and shrinkage thereof due to variations in an atmospheric temperature or repeated water absorption and drying. As a result, stress applied to a mounted portions of the wallboard at which it is mounted on the wall backings is relaxed such that an occurrence of cracks in the wallboard is prevented.
In the first method, however, each solid end of the external wallboard needs to have a predetermined strength. When the cement external wallboard has a thickness below 15 mm, the strength in each solid end of the wallboard is insufficient such that the earthquake force or wind pressure load tends to crack the solid ends of the wallboard. In view of this drawback, the external wallboard used in the first method needs to have a large thickness. Consequently, the cost of the wallboards is increased and the mounting cost is accordingly increased. Furthermore, dedicated fittings are required and further increase the mounting cost.
On the other hand, FIGS. 9A and 9B show the second method. As shown, an external wallboard 12 is applied to a wall backing 11 such as a C-steel and then fixed to the wall backing 11 by screws 13. The second method does not require any dedicated fittings, and the wallboard 12 can readily be mounted by less expensive screws 13. Furthermore, since the second method provides a large mounting strength, a relatively thin external wallboard can be used, and accordingly, the mounting cost can be decreased. However, the wallboard 12 is securely fixed to the wall backing 11 by the screws 13 in the second method. This hardly allows the wallboard 12 to be displaced relative to the wall backing 11. An excessive stress is concentrated on the mounted portions of the wallboard 12 when the earthquake force or wind pressure load causes the building framed construction to be displaced or when the variations in the atmospheric temperature or repeated water absorption and drying results in expansion and shrinkage of the wallboard 12. Consequently, there is a possibility that the external wallboard 12 may crack when subjected to the excessive stress concentrated on its mounted portions. The wall backing 11 is substantially a rigid body particularly when formed of a steel. In this case, a buffering characteristic and deformability as provided by a wooden wall backing cannot be expected from the steel wall backing. Consequently, an extremely large stress tends to concentrate on the mounted portion of the wallboard, resulting in occurrence of crack in the wallboard.